1. Field of the Invention
The present invention relates to a communication apparatus or device such as an optical communication apparatus/device (e.g., a light source or an optical transmitter) and a radio communication apparatus/device (e.g., a portable phone), and more particularly, to a High-Frequency (HF) circuit to be mounted on a communication apparatus/device of this type that prevents its HF characteristic from degrading, and a module equipped therewith.
2. Description of the Related Art
With optical communications systems, signal light is generated by direct or indirect modulation method of the output light of a semiconductor laser as a light source according to a signal to be transmitted in the transmission side. Then, the signal light thus generated is sent to an optical detector in the reception side by way of a medium.
In the direct modulation method of light, a semiconductor laser itself is driven with an intensity- or frequency-modulated current according to a signal to be transmitted, thereby generating the signal light. On the other hand, in the indirect modulation method of light, a semiconductor laser is driven with a dc current to generate output light with a constant intensity and then, the output light thus generated is modulated by an intensity-, frequency-, or phase-modulation method with an external modulator according to a signal to be transmitted, thereby generating the signal light.
As known well, when the bit rate of information is in the order of gigabits per second (i.e., Gb/sec) or higher, the transmittable distance is limited due to wavelength chirping occurring in the direct modulation method. Therefore, the indirect modulation method of light is used for transmission at such the high bit rate as above.
It is usual that an external modulator is formed in the form of module. For example, only an external modulator is formed as a module. Alternately, an external modulator and a light source (e.g., a semiconductor laser) are combined together to form a module, which is termed the xe2x80x9cmodulator-integrated light sourcexe2x80x9d module.
In recent years, the bit rate of information or data to be transmitted has been becoming higher (e.g., Gb/sec or higher). Therefore, to transmit the information from the transmission side to the reception side without any errors, there is the increasing need to improve the high-frequency characteristics of the external modulator. To meet this need, various improved modules of this type have been developed and disclosed.
FIGS. 1 and 2 show the configuration of an example of the prior-art modulator-integrated light source modules. As seen from these figures, on a conductive base 101, a dielectric substrate 102 and a heat sink 103 are formed to be apart from each other at a small distance. The dielectric substrate 102 is, for example, made of alumina (Al2O3). The sink 103 is made of material with high thermal conductivity.
A modulator-integrated light source chip 120 is mounted on the sink 103, where the chip 120 comprises a semiconductor laser 109 and an external modulator 110. The sink 103 serves to cool the chip 120, i.e., to dissipate the heat generated by the chip 120. The sink 103 is mechanically and electrically connected to the base 101 by way of a conductive via hole 111b. 
On the surface of the dielectric substrate 102, a patterned conductive layer is deposited, forming a strip-shaped signal line 107 and two ground lines 108a and 108b at each side of the signal line 107. The surface of the substrate 102 is exposed from the lines 107, 108a and 108b through two elongated windows. The signal line 107 and the ground lines 108a and 108b constitute a coplanar-type transmission line. The ground lines 108a and 108b are mechanically and electrically connected to the base 101 by way of conductive via holes 111a. The signal line 107 is not electrically connected to the base 101.
A matching resistor 104, which serves as a terminator for impedance matching, is formed on the exposed surface of the dielectric layer 102 between the signal line 107 and the ground line 108b. The resistor 104 is located near the end of the signal line 107, which is in the vicinity of the heat sink 103. The two ends of the resistor 104 are mechanically and electrically connected to the lines 107 and 108b, respectively. The resistor 104 is of the chip type or thin-film type.
The signal line 107 is electrically connected to the heat sink 103 by way of a conductive bonding wire 105. One end of the wire 105 is bonded to the nearer end of the line 107 to the sink 103. The other end of the wire 105 is bonded to the sink 103 at its nearest edge to the line 107. The heat sink 103 is electrically connected to the external modulator 110 of the chip 120 by way of a conductive bonding wire 106. One end of the wire 106 is bonded to the sink 103 at its nearest edge to the line 107. The other end of the wire 106 is bonded to the pad of the modulator 110 of the chip 120. The laser 109 is supplied with an electric, driving current by way of a conductive bonding wire 112.
A high-frequency electrical input signal SIN to be transmitted is applied to the signal line 107 from its furthest end from the heat sink 103. The signal SIN is then sent to the external modulator 110 of the modulator-integrated light source chip 120 by way of the signal line 107, the bonding wires 105 and 106, and the sink 103. The modulator 110 modulates the output light of the laser 109 according to the signal SIN thus inputted, generating the signal light. The signal light thus generated is emitted from the chip 120 and the modulator-integrated light source module.
With the prior-art module shown in FIGS. 1 and 2, as described above, the external modulator 110 of the modulator-integrated light source chip 120 and the dielectric substrate 102 are located to be apart from each other at a specific small distance, thereby decreasing the lengths of the bonding wires 105 and 106. Thus, the inductance components of the wires 105 and 106 are restricted, suppressing the degradation of the high-frequency characteristic of the modulator 110.
However, it is often that the distance between the dielectric substrate 102 and the modulator-integrated light source chip 120 is unable or difficult to be short as desired due to requirements in designing the module of this type. In other words, the bonding wires 105 and 106 are often unable to be short as desired. As a result, there is a limit in the method of preventing degradation of the high-frequency characteristic of the module by decreasing the lengths of the bonding wires. It is preferred that this problem is solved by a different method if possible.
In addition, the Japanese Non-Examined Patent Publication No. 10-275957 published in 1998 discloses an optical semiconductor chip carrier. This carrier comprises the same technique as described above while a microstrip line is used as the transmission line for the input signal SIN into the external modulator 110.
Accordingly, an object of the present invention is to provide a high-frequency circuit that makes it possible to prevent degradation of its high-frequency characteristic by a different method from the above-described method of decreasing the lengths of bonding wires used.
Another object of the present invention is to provide a high-frequency circuit module that makes it possible to prevent degradation of its high-frequency characteristic by a different method from the above-described method of decreasing the lengths of bonding wires used.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of she present invention, a high-frequency circuit is provided, which comprises:
(a) an electronic element having a capacitance;
(b) a signal line for transmitting a high-frequency electric signal to the element;
(c) a terminating resistor for impedance matching;
(d) a first bonding wire for electrically connecting the signal line and the element; and
(e) a second bonding wire for electrically connecting the element and the resistor;
wherein a characteristic impedance of combination of the element and the first and second bonding wires is equal to or greater than that of input side of the electric signal with respect to the combination;
and wherein an inductance of the second wire is greater than that of the first wire.
With the high-frequency circuit according to the first aspect of the present invention, the characteristic impedance of the combination of the electronic element and the first and second bonding wires is equal to or greater than the characteristic impedance of the input side of the high-frequency electric signal with respect to the combination. Also, The inductance of the second bonding wire is greater than the inductance of the first bonding wire. Therefore, the high-frequency characteristic of the high-frequency circuit according to the first aspect is prevented from degrading by the use of a different method from the above-described method of decreasing the lengths of the first and second bonding wires.
Needless to say, if at least one of the lengths of the first and second bonding wires is decreased, the above-described advantage of the circuit of the first aspect of the invention is enhanced.
In a preferred embodiment of the circuit according to the first aspect, the element has a conductive pad. The element and the first wire are electrically connected to each other at the pad while the element and the second wire are electrically connected to each other at the same pad.
In another preferred embodiment of the circuit according to the first aspect, the electronic element has a first conductive pad and a second conductive pad. The element and the first wire are electrically connected to each other at the first pad while the element and the second wire are connected to each other at the second pad.
In still another preferred embodiment of the circuit according to the first aspect, a conductive island electrically connected to the element by way of a third bonding wire is additionally provided. The signal line is electrically connected to the island by way of the first wire, thereby electrically connecting the signal line to the element by way of the first and third wires. The resistor is electrically connected to the island by way of the second wire, thereby electrically connecting the resistor to the element by way of the second and third wires.
In a further preferred embodiment of the circuit according to the first aspect, the element is a modulator for generating an electric or optical signal by modulation according to the electric signal transmitted through the signal line.
In a still further preferred embodiment of the circuit according to the first aspect, the inductance of the second wire is approximately twice in value the inductance of the first wire.
According to a second aspect of the present invention, a high-frequency circuit module is provided, which comprises a base and the high-frequency circuit according to the first aspect mounted on the base.
With the high-frequency circuit module according to the second aspect of the invention, the high-frequency circuit according to the first aspect is mounted on the base. Thus, there is the same advantage as that of the circuit of the first aspect.
In a preferred embodiment of the module according to the second aspect, the element is mounted on a heat sink fixed to the base and the signal line is located on a dielectric layer formed on the base. The resistor is fixed directly on the base.
In another preferred embodiment of the module according to the second aspect, the element is mounted on a heat sink fixed to the base and the signal line is located on a dielectric layer formed on the base. The resistor is fixed on the sink.
According to a third aspect of the present invention, a communication device is provided, which comprises the high-frequency circuit module according to the second aspect.